The use of power DC-DC converters and gyrator structures for energy processing in photovoltaic solar facilities 功率 DC-DC 转换器和回转器结构用于光伏太阳能设施中的能源加工

Transcription

1 ISSN Volume 1 (2014) issue 2, article 2 The use of power DC-DC converters an gyrator structures for energy processing in photovoltaic solar facilities 功率 DC-DC 转换器和回转器结构用于光伏太阳能设施中的能源加工 Herminio Martínez-García Barcelona College of Inustrial Engineering (EUETIB), Consorci Escola Inustrial e Barcelona (CEIB), Department of Electronics Department, Technical University of Catalonia (UPC) - BarcelonaTech, c/ Comte e Urgell, 187, 08036, Barcelona, Spain herminio.martinez@upc.eu Accepte for publication on 28th August 2014 Abstract - This article provies a classification of high efficiency switching power-gyrator structures an their use as cells for energy processing in photovoltaic solar facilities. Having into account the properties of these topologies presente in the article, their inclusion in solar facilities allows increasing the performance of the whole installation. Thus, the esign, simulation, an implementation of a G-type power gyrator are carrie out throughout the text. In aition, in orer to obtain the maximum power from the photovoltaic solar panel, a maximum power point tracking (MPPT) is manatory in the energy processing path. Therefore, the practical implementation carrie out inclues a control loop of the power gyrator in orer to track the aforementione maximum power point of the photovoltaic solar panel. Keywors DC-DC power converters, Power gyrators, Photovoltaic solar panels, Maximum power point tracking (MPPT). I. INTRODUCTION In general, photovoltaic subsystem panel o moule set (generation subsystem) oes not provie the same nominal voltage that the require by the output loa or battery (consume subsystem). Thus, in orer to aapt both generator an loa voltages, proviing at the same time high efficiency in this link, the use of a DC/DC power switching converter is manatory. As a consequence, switching converters are wiely use in photovoltaic generating systems as an interface or link between, on the one han, the photovoltaic subsystem (panel o moule set), an, on the other han, the loa an/or battery. In aition, it can allow the follow-up of the maximum power point (MPP) of the photovoltaic system in orer to obtain the maximum energy that this system can provie. Therefore, the main task of DC-DC converter in this kin of operation is to conition the energy generate by the array of panel following a specific control strategy [1]. Notice that the DC/DC conversion process implies, in turn, an associate effect of impeance transformation. In fact, the input impeance shows a epenence on a number of parameters such as loa resistance, uty cycle of the switching converter, etc. In this sense, converters are quite similar to classic transformers when they are use as impeance aaptors, except that in converters the aaptation parameter is not the turns ratio between the seconary an primary ones, but the uty cycle that controls the energy transfer. This uty cycle, as it is well known, can be governe electronically. In aition to the use of the aforementione switching power DC/DC converters, a maximum power point tracking (MPPT) is also highly recommene in the energy processing path in orer to obtain the maximum power from the photovoltaic solar panel. Therefore, normally, practical implementations inclue a control loop of the converter in orer to track the aforementione maximum power point of the photovoltaic solar panel. This maximum power point can be achieve thanks to the control of the switching converter uty cycle. This effect, which is the basis of MPPT systems,

2 also shows an o property: Certain input impeance values can be either reache or not, epening on the type of converter use, which significantly affects the performance of the photovoltaic system [1]. In DC/DC switching converters we have relation between average values of the input an output voltages via the uty cycle (or between average values of the input an output currents), proviing, at the same time, high efficiency. Therefore, we can say that they behave as high efficiency voltage-controlle voltage source (VCVS), or current-controlle current source (CCCS). Unlike classic DC/DC power converters, in power gyrators we have a relation between average values of the input voltage an output current (or input current an output voltage), also proviing, at the same time, high efficiency. Therefore, the key of this kin of converter is to obtain high-efficiency voltage-controlle current source (VCCS) or current-controlle voltage source (CCVS). The arrival of the switching semiconuctor evices in the ecae of the 1950s carrie out the appearance of switching converters an switching power supply systems. The major evelopment of DC-DC converters took place at the beginning of the ecae of 1960s, when switching semiconuctors were feasible an afforable evices, being applie in the aerospace inustry as one of their first uses. On the other han, the initial concept of gyrator is referre to networks with certain interesting properties. They are attractive for the synthesis of inuctive elements with properties nearer to the ieality than their counterparts of woun core. The concept of gyrator was introuce firstly by Tellegen in his paper The gyrator, a new electric network element publishe in 1948 [2], in which mention is mae for a network with unique properties, an was consiere as a new electrical network element ae to those alreay known. The treatment of the subject by Tellegen is rather theoretical an oes not venture into the practical esign of these elements, although formally foune their behavior an some of its properties. The term gyrator is, since then, use to call this kin of network, of which one of the first was introuce by the same Tellegen for a toroial ferromagnetic core woun at one en, an separate by a ielectric segment at the other. The introuction of the gyrator circuit concept in power processing (high-efficiency switching-moe power gyrators) is ue to Singer, who presente a particular gyrator in the circuits name POPI (power output=power input), escribing the ieal behavior of a particular switching power converter structure [3]. In 2005 it was shown that the gyrator circuits were unstable, an calculate the stable conitions necessary for its possible implementation [4], [5]. The article is organize as follows: In Section 2 an introuction to gyrator circuits an their classification are carrie out; Section 3 explains the use of power gyrators for energy processing in solar energy facilities, together with the use of a MPPT subsystem; Section 4 eals with the esign an implementation of a particular power gyrator implementation for the energy processing from a photovoltaic solar panel an its main simulation an experimental results. The article conclues with the main conclusions in Section 5. II. POWER GYRATOR CONCEPT The concept of power gyrator introuce in [3] relates to a general sort of circuits name POPI, escribing the ieal behavior of a switche-moe power converter. In general two big groups of power gyrators can be foun: G-type an R-type gyrators. A. G-Type Gyrator A power gyrator type G (Figure 1) is efine as a switching converter which satisfies equations (1) an with the characteristic that the input current an output current are not pulse. i gv ; i 2 gv 1, (1) 1 2 where the parameter g is the conuctance of the gyrator. The G-type power gyrator with controlle output current behaves like a current source in its output port. G-Gyrator (a) Fig. 1. G-type power gyrator: (a) Basic block, an (b) equivalent circuit. The general iea of a G-type power gyrator is to achieve a controlle epenent current source that epens on the input voltage an a gain factor g. If the parameter g is ajustable, a VCCS (voltage-controlle output current source) can be obtaine. The two-port G-type power gyrators that can be foun are of fourth orer, that is, the buck with input filter (BIF), the boost converter with output filter (BOF), the Ćuk converter an the Ćuk isolate converter, as illustrate, respectively, in figure 2. In the present work, a BIF G-type gyrator is use in orer to valiate the use of power gyrators for energy processing in photovoltaic solar facilities. B. BIF Gyrator The BIF G-type gyrator is a DC/DC switching converter, in particular a buck regulator with input filter. The BIF structure epicte in Figure 2 is an unstable system; therefore, its implementation may not be viable. In [5] it is emonstrate the nee of incluing a amping network to get the system to reach stability an how to calculate it. The propose circuit, incluing the propose stability network, is shown in Figure 3. The analysis of the BIF G-type gyrator controlle by means of a sliing control loop shows that the system must meet a series of inequalities or conitions to obtain the necessary stability of the circuit; in particular [5], [6]: (b) 63 Journal of Energy Challenges an Mechanics 2014

3 RC R C C C 1 2 (2) gr (3) 2 g RL1 g RR C g RL ( C C ) ( g C ) R D (4) Moules The simpler R-type power gyrators are shown in Figure 4. These converters are the boost with output filter (BOF) converter, the Ćuk converter, an Ćuk converter with galvanic isolation. Moules I g (a) Moules (a) Moules I g (b) Moules (b) (c) Moules 1:n (c) C O Fig. 4. Classification of R-type gyrators: (a) Boost with output filter (BOF), (b) Ćuk converter, an (c) Ćuk isolate. Moules 1:n () C O Fig. 2. Classification of G-type power gyrators: (a) Buck with input filter (BIF), (b) boost with output filter (BOF), (c) Ćuk converter, an () Ćuk isolate. Moules V1 i1(t) L1 Damping Network La Ra C1 C R VC Q1 D1 i2(t) L2 C2 Fig. 3. Buck with input filter (BIF) gyrator with amping network. C. R-Type Gyrator A power R-type gyrator is efine as a switching converter with a switch topology characterize by (5): v ri ; v2 ri1, (5) 1 2 where r is the resistance implemente by the gyrator. Iout RL V2 From these ifferent structures, the more use is the first one (the BOF converter). III. POWER GYRATOR FOR THE ENERGY ACQUISITION OF A PHOTOVOLTAIC PANEL In this section, in orer to emonstrate the feasibility of eveloping power gyrator structures for solar energy applications, the esign an implementation of a BIF G-type power gyrator are carrie out. The objective is to process the energy provie by a photovoltaic panel. The panel use for the implementation of the application is supplie by the company BP. This is a panel that consists of 36 high-efficiency photovoltaic polycrystalline cells, proviing a maximum power of 10 W, an open circuit voltage (V OC ) of 21 V, an a short-circuit current (I SC ) of 0.65 A. Regaring the battery, that acts as the output loa, it must be note that the typical value of series resistance is 0.11 Ω. This, together with the panel, will establish the esign specifications of the implemente power gyrator. The initial esign specifications are presente in Table I. 64 Journal of Energy Challenges an Mechanics 2014

4 Fig. 5. Schematics of the implemente BIF G-type gyrator. TABLE I. INITIAL DESIGN SPECIFICATIONS Vin=20 V Iin=0,6 A ΔIout=5% Vout=12 V Iout=1 A ΔVout=0,5% RL=0,11 Ω The esign of the power gyrator is ivie into two parts: On the one han, the calculation of the components of the buck regulator, an, on the other, the other elements of the whole power gyrator structure, such as the filter input an the network stability. A. Component Design of the Buck Converter Assuming that, as esign specifications, the converter operates in continuous conuction moe (CCM), that it has a nominal input voltage of 20 V, an is esire an output voltage equal to 12 V an a maximum loa current of 1 A, the nominal uty cycle converter shoul be 60%. Once obtaine the uty cycle, the values of the require inuctance an capacitor can be obtaine, setting an output current ripple equal to 5%, an output voltage ripple of 0.5%, an a switching frequency equal to 50 khz. With these esign specifications, an inuctance of 1.64 mh is use, obtaining an output current ripple of 5.85%. Finally, the stanar value for the capacitor is C=3.3 μf, achieving a ripple voltage equal to 0.37%, which is perfectly suite to the level require for the voltage ripple at the converter output terminals. B. Component Design of the BIF Power Gyrator As alreay mentione, a G-type power gyrator has a variable g, an LC input filter, an a stable network whose parameters shoul be calculate to obtain the appropriate values for the provie initial conitions. To obtain g, the above assume initial conitions for currents an voltages at the converter input an output terminals are use, set g= Calculating the input filter, it carries out to a stanarize value of 22 μh for the inuctance, an a value of 1 μf for the capacitor. As iscusse above, G-type power gyrator structures require a stability network for ensuring their proper performance. This stability is achieve by a capacitor connecte in series with a resistor, resulting in a 3.3 nf capacitor (C ) an a resistance R =2 kω. These values are necessary in orer to etermine the stability network. In orer to ensure the stability of the system, whose equations are etermine by expressions (2), (3) an (4), these three expressions shoul be fulfille with the obtaine component values for this particular BIF G-type gyrator. On the other han, the necessary control law (in this case a sliing control) for the proper operation of the G-type gyrator shoul be establishe. Basically, this analog controller consists of a current sensing system comprising a shunt resistance of 50 mω, a ifference amplifier, a multiplier (an AD633 from Analog Devices) for the prouct of the input voltage establishe by the parameter g, an a comparator with some hysteresis to fix the switching frequency. Finally, the use of a PIC microcontroller (in this case, a Microchip s PIC18F1220) achieves the tracking of the PV-panel MPP. In the case carrie out in this article, the MPPT algorithm implemente has been the well-known perturb an observation (P&O) [1]. Notice that, for this tracking, it is necessary a secon current an voltage sensing, in orer to measure an introuce them to the PIC microcontroller. The output of the PIC responsible for proviing the value for the parameter g shoul be an analog value; however, the PIC can only offer at its output binary states (5 V or 0 V). To overcome this problem, an RC filter is 65 Journal of Energy Challenges an Mechanics 2014

5 Fig. 6. Experimental efficiency of the G-type power gyrator for ifferent values of the input current. ae, in orer to filter the switching frequency of the output voltage an provie an average voltage that controls the power gyrator. the G-type power gyrator, for ifferent input currents, is shown in Figure 7. IV. IMPLEMENTATION, SIMULATION AND EXPERIMENTAL RESULTS OF THE FINAL IMPLEMENTED G-TYPE GYRATOR PROTOTYPE (%) Efficiency (%) Figure 5 shows the complete schematic of the converter BIF carrie out in this work. In orer to corroborate the proper operation of the esigne an implemente BIF G-type gyrator, it has been powere by a solar panel. Therefore, the value of its parameter g is not fixe or manually ajustable, but the PIC microcontroller will be in charge of making the perturbation algorithm an monitoring to ajust the parameter g in orer to always assure (inepenently of the irraiance conitions) the maximum power of the solar panel (MPPT). The final characteristics obtaine from the implemente system are: Minimum input voltage equal to 18 V an nominal 20 V; an, output current ajustable between 1 A an 2.5 A, regarless of the value of the loa connecte, as long as the prouct of the output current an the loa resistance is lower than the input voltage. Finally, figure 6 shows a photograph of the complete implementation performe. For an input of 18 V, a reference voltage V g whose value forces an input current of 1.9 A, an an output 4.7-Ω loa, an output voltage an current equal to 11.8 V an 2.4 A, respectively, are obtaine. For the prototype of the G-type power gyrator hel for processing power of the photovoltaic panel, ifferent lighting conitions for the solar panel were use by means of ifferent points of light. After verifying their behavior as G-type power gyrator, an experimental analysis of the efficiency was carrie out for ifferent values of input current (setting the value g), an maintaining the value of the loa equal to 4.7 Ω (Table II). The experimental efficiency of I in (A) Fig. 7. Experimental efficiency of the G-type power gyrator for ifferent values of the input current. This graph shows how, as the G-type power gyrator works closer to the optimum point (I in =2 A), the performance an efficiency are enhance. Note that the efficiency of the gyrator is etermine by the value of the loa, an that, for a given output current value, ifferent output voltages can be obtaine. Finally, in figure 8, we can appreciate output current response when the reference voltage that ajusts the parameter g has an increasing step from 25 mv to 50 mv at t=25 ms, an when the input voltage suffers a new increasing step from 20 V to 22 V at t=50 ms. In both cases, notice that the output current is moifie accoring to equation (1). 66 Journal of Energy Challenges an Mechanics 2014

6 V in (V) V g (mv) (A) t (ms) Fig. 8. Simulation results of the implemente G-type power gyrator. TABLE II. EXPERIMENTAL RESULTS TO OBTAIN THE EFFICIENCY OF THE G-TYPE POWER GYRATOR IMPLEMENTED Iin (A) Vout (V) Iout (A) Pin (W) Pout (W) η (%) V. CONCLUSIONS This paper has provie, on the one han, a classification of high efficiency switching power-gyrator structures an, on the other, the valiity of their use as cells for energy processing in photovoltaic solar installations. In particular, having into account the properties of these topologies presente in the article, their inclusion in solar facilities allows increasing the performance of the whole installation. The esign, simulation an implementation of a G-type power gyrator are carrie out throughout the article, incluing a sliing control implemente by means an analog controller. In aition to the use of the aforementione switching power gyrator, a maximum power point tracking (MPPT) is manatory in the energy processing path in orer to obtain the maximum power from the photovoltaic solar panel. Therefore, the practical implementation carrie out inclues a control loop of the power gyrator in orer to track the aforementione maximum power point of the photovoltaic solar panel. In the presente esign, this MPPT circuit has been implemente by means of a PIC microcontroller, a Microchip s PIC18F1220, that achieves the tracking of the PV-panel MPP. In the case carrie out in this article, the MPPT algorithm implemente has been the aforementione perturb an observation (P&O). ACKNOWLEDGMENT This work has been partially fune by project TEC /MIC from the Spanish Ministerio e Ciencia e Innovación (MICINN) funs. REFERENCES [1] J.M. Enrique, E. Durán, M. Sirach-e-Carona, an J.M. Anújar. Theoretical Assessment of the Maximum Power Point Tracking Efficiency of Photovoltaic Facilities with Different Converter Topologies, Solar Energy, Vol. 81 (n. 1), pp , Jan [2] D. H. Tellegen. The Gyrator, a New Electric Network Element, Philips Research Reports, Vol. 3, pp , Apr [3] S. Singer, an R. W. Erickson. Canonical Moeling of Power Processing Circuits Base on the POPI Concept, IEEE Transactions on Power Electronics, Vol. 7 (n. 1), pp , Jan Journal of Energy Challenges an Mechanics 2014

7 [4] A. Ci-Pastor, L. Martinez-Salamero, C. Alonso, J. Calvente, an G. Schweitz. Synthesis of PWM-Base Power Gyrators, Proceeings of the IEEE International Symposium on Inustrial Electronics (ISIE 2005), Vol. 3, pp , Jun [5] A. Ci-Pastor, L. Martinez-Salamero, C. Alonso, B. Estibals, J. Alzieu, G. Schweitz, an D. Shmilovitz. Analysis an Design of Power Gyrators in Sliing-Moe Operation, Proceeings of the IEE Electric Power Applications, Vol. 152 (n. 4), pp , 8 Jul [6] A. Ci-Pastor, L. Martinez-Salamero, C. Alonso, A. El Aroui, an H. Valerrama-Blavi. Power Distribution Base on Gyrators, IEEE Transactions on Power Electronics, Vol. 24 (n. 12), pp , Dec Journal of Energy Challenges an Mechanics 2014